U.S. Pat. No. 4,018,685 discloses proportional valve switching for gradient formation. U.S. Pat. No. 4,595,496 discloses a liquid composition control for avoiding pump draw stroke non-uniformities. U.S. Pat. No. 4,980,059 discloses a liquid chromatograph. U.S. Pat. No. 5,135,658 discloses a coordinated chromatography system. U.S. Pat. No. 7,631,542 discloses a chromatography system with fluid intake management. U.S. Pat. No. 5,862,832 describes a gradient proportioning valve. International patent application WO 2010/030720 discloses a modulation of time offsets for solvent proportioning.
EP 1,724,576 discloses methods for operating a chromatography system. Specifically, methods for operating a pumping system within a chromatography system are disclosed. The methods presented are directed to pumping apparatus in which very small amounts of fluid are pumped through the system. A number of techniques for detecting and compensating for leaks are presented as well as a number of techniques for operating the pumping system.
U.S. Pat. No. 4,624,625 discloses that a high pressure metering pump has a duty cycle consisting of an aspiration portion where liquid is aspirated into a pumping chamber, a compression portion where the aspirated liquid is compressed to feed pressure, a feed portion where a part of the compressed liquid is expelled out of the pumping chamber, and a decompression portion where the liquid remaining in the pumping chamber is expanded to aspiration pressure. A measurement and control apparatus for the pump comprises a controller for adjusting and keeping constant the mean flow rate of the pumped liquid on the aspiration side or on the high pressure side of the pump. The apparatus further comprises a detector for detecting the transition point between the compression and feed portions and/or between the decompression and aspiration portions. The detector derives a control signal for the pump speed and for the optimal opening instant of an externally actuated input valve of the pump from the phase relationships of said transitions.
U.S. Pat. No. 4,128,476 discloses that in the cycle of a positive displacement LC pump system the output pressure is sensed to measure onset of output flow as a time lag from the beginning of the output stroke to give an estimate of the input filling time lag from the beginning of the fill stroke. This estimate is used to recompute a proportional module output for controlling the timing of a proportioning valve at the inlet to the LC pump so as to bring the actual operating conditions of the pump fill cycle into correspondence with the predetermined demanded ratios of the components either in isocratic or gradient program mode.